Expert: Zlatko - 3/12/2011

Question
I am trying to convert a code which runs on windows to run on linux. Part of the code takes a string and writes it to a file. In windows I do this using this bit of code:

file.open ("Highscore.txt", fstream::in | fstream::out | fstream::app);

file.write(test,500);

file.close();

however this doesn't create or link to a file in linux. Is there another way to do this that will work in linux.

Answer
Sam, I don't think this is a Linux issue. If the file does not already exist, then the only way file.open will succeed with fstream::in, is if you specify fstream::out and fstream::truc as well. However, I suspect that you do not want to trunc an existing file, so I suggest doing the open in 2 attempts, first with
in | out | ate, and if that fails, because the file doesn't exist, then use in | out | trunc.

If the file exists, the in | out | ate mode will succeed, and the file pointer will be set to the end of the file for appending. Here is sample code. I hope it helps.

Best regards
Zlatko



#include <iostream>
#include <fstream>

using namespace std;
int main(void)
{
   fstream file;

   // Notice I am using fstream::ate here, not fstream::app

   file.open("test.txt", fstream::in | fstream::out | fstream::ate);
   if ( ! file.good())
   {
       file.clear(); // It is important to clear the error before the next operation

       file.open("asd.txt",  fstream::in | fstream::out | fstream::trunc);
       if ( ! file.good())
       {
           cerr << "Cannot open file\n";
       }
       else
       {
           cerr << "Open file OK\n";
       }
   }
}